Recently, development of ships, automobiles, etc., which are operated to run by electrically driving motors, has been progressed with the view of addressing depletion of fuel sources, such as gasoline, and environmental deterioration caused by exhaust gases. In particular, by employing a superconducting motor disclosed in Japanese Unexamined Patent Application Publication No. 6-6907 (Patent Document 1), an ohmic loss in a superconducting coil can be eliminated and efficiency can be increased. Further, the motor size can be reduced and the motor output can be increased.
Superconducting structures using superconducting wires have also become increasingly prevalent in power generators, transformers, etc., in addition to motors.
However, if a magnetic field acts on a superconducting wire, characteristics of the superconducting wire deteriorate, whereby a large current cannot be supplied through the superconducting wire. In a structure that a superconducting coil is attached to an iron core, particularly, a magnetic field generated by energization of the superconducting coil is strengthened by the iron core and acts on the superconducting coil itself. Therefore, a current capable of being supplied to the superconducting coil is reduced and a current density is also reduced. This leads to the problem that the size of the superconducting coil and hence the size of a superconducting device have to be increased in order to supply a desired amount of the current.
That problem will be described in more detail in connection with a C-type magnet (iron core) to which a superconducting coil is attached.
As shown in FIG. 10, a superconducting coil 2 is formed by winding a superconducting wire over a C-type iron core 1 at a desired position with no gap left between the superconducting coil 2 and the C-type iron core 1, and another magnetic material 5 is arranged in a gap 1a of the C-type iron core 1. The magnetic material 5 may also be formed of an iron core. When a current is supplied to the superconducting coil 2, magnetic fluxes F1 and F2 are excited, for example, as indicated by broken lines. The magnetic flux F1 passes through the C-type iron core 1 and generates a magnetic field in the gap 1a, thereby magnetizing the magnetic material 5 arranged in the gap 1a. On the other hand, the magnetic flux F2 passes through air around both the C-type iron core 1 and the superconducting coil 2 in the vicinity of the superconducting coil 2 without passing through the gap 1a. The magnitude of magnetic flux is represented by “magnetomotive force/magnetic resistance”. Therefore, if the magnetomotive force is constant, the magnitude of magnetic flux is increased as the magnetic resistance is reduced. This means that the magnetic flux F2 becomes relatively strong because the magnetic flux passes through not only air having a large magnetic resistance (low magnetic permeability), but also the C-type iron core 1 having a small magnetic resistance (high magnetic permeability). As a result, the intensity of a magnetic field acting on the superconducting coil 2 is increased and characteristics of the superconducting coil 2 deteriorate.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-6907.